Exodus 7:14-18

Then the LORD said to Moses, “Pharaoh’s heart is stubborn; he refuses to let the people go. Go to Pharaoh in the morning as he is going out to the water, and station yourself to meet him on the bank of the Nile; and you shall take in your hand the staff that was turned into a serpent. You shall say to him, ‘The LORD, the God of the Hebrews, sent me to you, saying, “Let My people go, that they may serve Me in the wilderness. But behold, you have not listened until now.” Thus says the LORD, “By this you shall know that I am the LORD: behold, I will strike the water that is in the Nile with the staff that is in my hand, and it will be turned to blood. The fish that are in the Nile will die, and the Nile will become foul, and the Egyptians will find difficulty in drinking water from the Nile.” ’ ”